Since the end of 1980s, when Baibich et al discovered the Giant magneto Resistance (GMR) in a magnetic multilayer film system for the first time, the research of magnetic multilayer film system has gain wide attention from scientific researchers. Since GMR effect has very high magnetoresistance ratio, it can be widely applied in areas such as magnetoresistance type sensors, magnetic recording read head etc. The devices based on the GMR effect are not only with excellent characters such as high sensitivity, small volume, and low power consumption etc., but also bring many new characters such as anti-radiation, nonvolatile information storage etc. Especially the application of GMR effect into magnetic recording read head has brought a profound revolution to the entire information recording area, and has produced direct and far-reaching effect to related industries. In 1994, IBM developed read head for hard disk utilizing GMR effect, which increased the recording density of the magnetic disk storage system nearly 20 times, which is a breakthrough for the computer industry; the design of various sensory devices based on GMR effects are greatly simplified due to the increase of output signals, which directly leads to the miniaturization and cheapness of the devices.
After the discovery of GMR effect, in 1995, a Japanese scientist T. Miyazaki and a U.S.A. scientist J. S. Moodera acquired a Tunneling Magneto Resistance (TMR) ratio of 18% and 10% at room temperature in a Magnetic Tunnel Junction (MTJ), respectively, which generated the research heat of MTJ. The researchers, based on GMR effect and MTJ, have designed a device model of a new type of Magnetic Random Access Memory (MRAM), which, due to totally new designs, has many new exciting features, such as anti-radiation, non-volatile information storage etc. In a typical design of MRAM device, the core structure includes four parts: bit line, word line, read line and memory cell. The bit line and word line, read line are positioned above and below the memory cell respectively, crossing each other, and the memory cells are located at the cross section of bit lines and word lines. The writing procedures of MRAM are completed by the reverse of magnetic moment of the bit layer, which is driven by the combined magnetic field co-produced by the two pulse currents flowing through the word line and the bit line. Therefore, such working way depends obviously on an intermediate step, the magnetic field produced by the two pulse current of word line and bit line, to control the magnetic states of a memory cell, which makes the structure and reparation process very complicated, and cause great inconvenience and pretty high cost to the processing and integration of MRAM devices.
In 1996, a U.S.A. scientist J. Slonczewski predicted a new physical mechanism—Spin Torque (ST) effect, theoretically, which utilizes current itself to realize the control of magnetic states of a memory cell. When the current flowing through a memory cell is lower than certain specific threshold value Ic, the magnetic states of the memory cell will not be changed by the current flowing through the memory cell, by which the read operation can be realized; when the current flowing through a memory cell is higher than the threshold value Ic, the magnetic states of the memory cell is determined by the direction of the current flowing through the memory, by which the write operation can be realized. For the next more than ten years, scientists have conducted large amount of open and deep research of this new effect. If the new mechanism can be applied into devices such as magnetic multilayer system and MRAM, the device structure and processing technique will be greatly simplified, leading to another revolutionary breakthrough for the area of information storage.
However, because the geometry structure of the memory cells used in prior art—such as bit layer (soft magnetic layer) and other pinned magnetic layer (or hard magnetic layer)—has adopted non-close structure such as rectangle, eclipse, etc. This kind of structure brings relatively larger demagnetization field and shape anisotropy in a memory cell with high density and small scale, which, beyond all doubt, increases the adverse field (coercive force) and power consumption of the bit layer (soft magnetic layer). At the same time, the magnetic coupling and interferences among memory cells are unavoidable in the high density state, which bring many disadvantageous effects and magnetic noises to the uniformity and consistency of the electromagnetic features of the memory cells, and also the complexity of the structure and processing techniques to the design and fabrication of the memory cells. Currently, to decrease the demagnetization field, the MTJ is used as a memory cell, the upper and lower magnetic electrode of which have adopted the bit layer and bottom pinning layer of manual pinning compound type with sandwich style (e.g. Co—Fe/Ru/Co—Fe—B and Py/Ru/Co—Fe—B). However, the adverse field and power consumption of its bit layer fails to lower to a perfect and desired minimum value. To overcome these problems, new geometry structures and principle for designing a device should be adopted to eliminate the demagnetization field produced by the memory cells themselves when the magnetic multilayer has experienced down scale patterning of micro-fabrication and nano-fabrication, and to further decrease the shape anisotropy of bit layers of the memory cells.